Fuel cells that generate electric power through electrochemical reactions between hydrogen and oxygen are drawing attention as an energy source. A fuel cell has a construction in which an electrolyte is sandwiched between a hydrogen electrode and an oxygen electrode. When the hydrogen electrode is supplied with a hydrogen-rich fuel gas and the oxygen electrode is supplied with an oxidizing gas, such as air or the like, hydrogen and oxygen from these gases react to generate electric power.
Some fuel cell-equipped electric vehicles use a secondary battery in addition to a fuel cell, and perform a control of selectively using them in an appropriate fashion. For example, in a technology disclosed in Japanese Patent Application Laid-Open Publication No. 2001-307758, both a fuel cell and a secondary battery are used to supply power to an electric motor provided as a drive power source during a normal condition. During an operation state where the power generation efficiency of the fuel cell becomes low, a control of stopping the power generation of the fuel cell and driving the electric motor via only the secondary battery is performed.
However, in the above-described fuel cell system, a considerable amount of time is needed for supplying the oxidizing gas or the fuel gas, and activating the electrochemical reactions, and raising the voltage to a required level at the time of a change of control of power generation of the fuel cell from a stopped state to an operating state. Therefore, in some cases, a requested electric power cannot be smoothly output, but a delay occurs in electric power generation. Such a power generation delay sometimes causes degraded drivability in the case of a mobile unit, such as a motor vehicle or the like. Of course, this problem also concerns standstill-type fuel cell systems and the like.